This invention relates generally to ultrasonic flowmeters for measuring the flow rate of fluids passing through a flow pipe, and more particularly to a meter of this type capable of accurately and reliably measuring the flow rate of industrial process fluids which range in quality from clear to moderately dirty.
In general, ultrasonic flowmeters are either of the through-beam or of the Doppler-type. In a through-beam meter, the meter pipe has upstream and downstream transducers mounted thereon which are alternately excited at a relatively rapid rate to generate an ultrasonic pulse which is propagated through the fluid being metered and received by the other transducer. The upstream propagation time minus the downstream propagation time represents the time delay difference between the generation of the emitted pulses and their reception. This difference is a function of the flow velocity of the fluid and is convertible into a flow rate reading. Among the patents which disclose through-beam ultrasonic flowmeters are U.S. Pat. Nos. 4,103,550, 4,004,461, 3,906,791 and 4,195,517.
The through-beam ultrasonic flowmeter is acceptably accurate only when the liquid being metered is substantially free of contaminants; for the presence of particulate matter in the propagation path causes this meter to malfunction. In contradistinction, contaminants in the fluid are essential to the operation of a Doppler-type meter.
The Doppler effect is encountered whenever a wave source generating sonic, radio-frequency, light or any other form of wave activity and a wave receiver are in relative motion with respect to one another. When the distance between the source and its receiver is decreasing, extra waves are detected in a given time, resulting in an apparent increase in the received frequency. When, however, the distance is increasing, an apparent decrease is experienced in the frequency of the received signal.
In a Doppler-type ultrasonic flowmeter, a continuously transmitted signal is mixed with the received signal to produce a beat signal which represents the difference therebetween, the frequency of the beat signal being proportional to the velocity of the reflecting contaminants and hence to the flow rate of the fluid through the flow pipe.
Because in a Doppler-type ultrasonic flowmeter the transmitting and receiving transducers are mounted on the exterior of the meter pipe, the instrument is obstructionless and is free of erosion and corrosion problems. The normal fields of application for a Doppler-type flowmeter are pipes conducting contaminated media where through-beam flowmeters do not work or perform poorly. Thus Doppler-type ultrasonic flowmeters are useful in metering raw sewage or sludge in water treatment plants as well as in metering wash or waste-water, slurries and effluents in chemical, paper processing and mining systems. As noted in the article by Morris in the August 1979 issue of Control Engineering, entitled "What's Available in Ultrasonic Flowmeters," the accuracy of Doppler-type flowmeters is normally less than that quoted for through-beam meters.
Some industrial process fluids are neither sufficiently clean to permit flow measurement with through-beam ultrasonic meters, nor adequately contaminated with reflective particles to allow for measurement with Doppler-type meters. Hence with existing types of ultrasonic flowmeters, it is difficult to obtain reliable readings with fluids which vary from a relatively clean state to a moderately dirty condition, for no one form of available meter is capable of responding effectively to both states. Moreover, the response time of existing meters is relatively sluggish.